This invention relates to soluble packages such as pouches containing one or more products and to apparatus and method for producing such pouches. The invention has particular, but not exclusive, application to thermoformed packages made from two or more webs comprising polymeric substrates. The webs are soluble or dispersible in a solvent and may comprise water soluble or water dispersible films.
Thermoformed packages comprising a single compartment are typically made in four steps:                1. Forming a pocket in a first web of film (the base web)        2. Filling of the pocket so formed        3. Closure of the pocket by means of a second web of film (the top or lidding web) to form a package.        4. Cutting or punching to release the package from the combined first and second webs of film        
Thermoformed packages are frequently made on intermittent horizontal machines in which the web is moved through the machine in intermittent steps allowing each operation described above to be carried out whilst the web or webs are stationary.
In such machines, once the base web is moved away from the forming mould, it begins to cool and unlike many insoluble polymeric substrates, many soluble webs, including particularly those comprising substantially polyvinyl alcohol, then experience a phenomenon known as shrink-back in which the thermoformed film tries to return to its pre-thermoformed condition thereby reducing the volume of the thermoformed pocket. This represents a major problem as the pocket will have reduced in size by the time that it is filled. To take account of the shrink-back and still maintain the required fill volume, the thermoformed pocket needs to be made larger than the fill volume so that it can accept the fill volume despite the effects of shrinkback. Designing a thermoforming mould with larger cavities requires either a deeper draw depth, which in turn often requires a thicker base web provoking slower subsequent dissolution, or a greater superficial area, known as the footprint, allowing fewer cavities in either the machine direction or across the web or both, thereby reducing machine and web material efficiencies.
A continuous motion rotary thermoforming machine for making thermoformed packages is disclosed in U.S. Pat. No. 3,218,776 in which the base web is heated to a plastic condition and immediately pressed on to the surfaces of the drum surrounding the pockets. A heated metal roller is employed to press the web of film against the surfaces of the drum. The roller is heated to a temperature sufficient to render the film plastic by the time that it is pressed against the continuously rotating drum. The actual temperature will vary depending upon the film, its thickness, and the length of time that it remains in contact with the heated roller. The film, by means of the heated roller, may be heated to about 140° F. (60° C.) but it might be as low as about 100° F. (37.8° C.) or as high as 300° F. (148.9° C.). A vacuum is applied to the base of the cavity in order to draw the web of heated film that covers the cavities into the cavities and define pockets to receive subsequently the product to be packaged. The vacuum should be applied instantly after the cavity is covered with film. In a subsequent operation, the top web is pressed against the base web, by now containing filled thermoformed pockets located upon the rotating drum, by means of a second heated roller which conductively heats the top web and, by means of a spring, presses the top web over the pockets containing the packaged product during which both webs are pressed together on the surfaces surrounding the pockets to achieve a bond. In the particular case where water-soluble packages are desired, the top web, prior to arrival at the sealing station, is made adhesive by moistening with a liquid in which the film of the top web is soluble, in order that the sealing process can be effected by a combination of heat and solvent welding.
In the machine disclosed in U.S. Pat. No. 3,218,776, two heated rollers provide the sole sources of heat, a first heated roller heating the base web prior to thermoforming and a second heated roller heating the lidding web prior to closure of the pouches. This has a number of disadvantages. Firstly, the base web begins to cool as soon as it leaves the first heated roller and a high vacuum is therefore required to hold the base web to the walls of the cavities and thereby inhibit the occurence of shrink-back. Secondly, the top web begins to cool as soon as it leaves the second heated roller, thereby preventing the strongest seal from being made between the base web and the top web.
Continuous motion rotating thermoforming offers advantages compared to intermittent motion horizontal thermoforming, particularly in respect of higher productivity and reduced shrink-back. Due to the higher productivity of continuous motion rotating machines, the time interval between thermoforming and filling is reduced, thereby allowing the thermoformed web less time to cool. Nevertheless, shrinkback of soluble webs has remained a problem, even with continuous motion rotary machines such as described in U.S. Pat. No. 3,218,776.